Cashew nut shell liquid (CNSL) is extracted from the outer shell of the cashew nut obtained from the cashew nut tree, Anacardium Occidentale. In the shell CNSL comprises mainly anacardic acid (2-carboxy-3-pentadecadientyl phenyl) and related acids of different degrees of unsaturation, together with cardol (m-pentadecadienyl resorcinol). The liquid is extracted from the shells by a heating process which results in decarboxylation of the acids so that the material obtained consists of cardanol (I) and cardol (II) and related compounds of varying degrees of unsaturation. A typical analysis of technical CNSL is shown below: ##STR1## The side chains exist in saturated (N=0), monoene (n=2), diene (n=4) and triene (n=6) forms with cis configuration.
The components of CNSL contain a phenolic hydroxyl group and an unsaturated side-chain giving the material a dual functionality in polymerization reactions. As well as reacting with aldehydes to give products analogous to conventional phenol formaldehyde resins, CNSL can also be polymerized via the unsaturated side-chain. This versatility has led to the use of CNSL in various industries. Historically the main outlet for CNSL has been in the manufacture of brake linings and clutch facings where it is compounded with asbestos. Other applications have included cements and adhesives, laminated board manufacture and insulating varnishes.
The polymerization products of CNSL are generally dark brown in color and therefore the application of CNSL in the surface coating industry has been limited to those situations in which color is relatively unimportant.
It has been established that the dark colors formed during the polymerization of CNSL are attributable to the presence of polyhydric phenols, primarily cardol. Therefore in order to obtain colorless or lightly colored resins suitable for use in surface coatings, it is necessary to use CNSL which is substantially free of polyhydric phenols, namely cardol.
Various methods of purifying CNSL are known. Technical CNSL can be distilled under high vacuum with wide bore distillation equipment or by molecular distillation to yield a cardanol/cardol mixture possessing a pale yellow color as distillate and a brown material as the residue. Upon standing, some reversion of color of the distillate does occur. The recovery of phenolic material is generally approximately 70%. A slight loss of cardanol occurs in the distillation but generally all fractions contain both cardanol and cardol and separation of the two is not obtained.
Pure cardanol, free of cardol, has been obtained by column chromatography, but this method is expensive and time consuming and it is not suitable for use on an industrial scale.
It is an aspect of the present invention to describe a method of treating CNSL to reduce the content of dihydric phenols therein.